1. Field of the Invention
The present invention relates generally to material handling equipment, and more particularly to an apparatus for transporting semiconductor wafers through a loadlock into vacuum processing equipment.
Loadlocks, also referred to as airlocks, are routinely employed in a variety of circumstances where material must be moved from an area at a first pressure to a different area at a second pressure. The loadlock is a sealable chamber where the internal pressure can be adjusted to match the outside pressure found at one or more ports. Of particular interest to the present invention are loadlocks which allow transport of semiconductor wafers from a first pressure, typically ambient, to processing equipment operating at very low pressures, such as plasma etchers and the like.
Operation of the loadlock requires a mechanism for retrieving the material from the outside, transporting the material to the interior of the airlock, supporting the material while the airlock is evacuated, and delivering the material to the desired processing vessel when the pressure has been equalized. A variety of systems, normally employing manipulable arms, have been developed, and some of these systems are described in detail in the Prior Art cited hereinbelow. The prior art systems, however, suffer from a number of disadvantages which limit their usefulness and require relatively frequent maintenance. A major disadvantage of the systems is the presence of exposed joints, bearings, hinges, and the like, which require lubrication. The lubricating fluid applied to such mechanical connections quickly vaporizes in the low pressure environment, requiring frequent reapplication and in the worst case failure of the system. More importantly, such exposed joints generate particulates each time the joint is operated, and the particulates will frequently deposit on the wafers being transported. Particulates on the wafer surface can degrade the product which is eventually produced.
A second disadvantage of the prior art systems has been the large size of the transfer assemblies. While it is desirable to minimize the volume of the loadlock in order to reduce the time and power required to draw a vacuum therein, the size of the transfer assembly frequently requires enlargement of the loadlock.
Additionally, the prior art transfer mechanisms frequently do not allow for minor adjustments to the pickup and discharge locations, such as those which are frequently required because of misalignment of the loadlock with the processing equipment. Such systems are often also limited mechanically to transport along one or two preselected patterns, and cannot be later adjusted to provide for different delivery geometries.
For these reasons, it would be desirable to provide a workpiece transfer assembly suitable for operation in very low pressure environments and having few, if any, exposed lubricated joints. It would be further desirable if the mechanism is adapted so that the majority of moving parts lie outside of an associated loadlock, and that the mechanism be adjustable and provide for a variety of different transport patterns and easy correction of minor misalignments.
2. Description of the Prior Art
Of particular interest to the present invention are U.S. Pat. Nos. 4,433,951 and 4,483,654, which relate to a wafer transport mechanism employing a rotatable, articulated arm. The mechanism includes numerous lubricated joints which are exposed to the vacuum inside a loadlock. A number of U.S. and foreign patents disclose alternative mechanisms for transporting wafers to and from vacuum processing equipment, see, e.g., U.S. Pat. Nos. Re. 25,889; 3,656,454, and 3,874,525; British Pat. Nos. 1,395,058; 1,570,066; 2,022,047A; and 2,121,747A, and Japanese Pat. 55-116432.